[time-nuts] filtering a 10Mhz frequency standard?

Bob Camp lists at rtty.us
Tue Dec 13 12:19:12 EST 2011


Hi

Regardless of the design, what ever filter you use will be sensitive to
load. Hooking up a few dozen instruments to a standard line with BNC T's is
unlikely to present 50 ohms to the filter. The more complex the filter,
generally the more sensitive it will be ....

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of ed breya
Sent: Tuesday, December 13, 2011 12:07 PM
To: time-nuts at febo.com
Subject: [time-nuts] filtering a 10Mhz frequency standard?

I reported a filter design I was working on back in November, using 
10 base T LAN filter modules. The first experimental air-wired unit 
with two modules got to about -75 dBc at 30 MHz, rising to -65 dBc at 70
MHz.

After building the same circuit with shielded compartments, it ran 
about -85 dBc flat over this range, but it should have been over 100. 
The weak link turned out to be the cross-talk within the modules 
themselves - there are two 17 MHz LPF sections in each, and I 
cascaded them all.

Four separate modules cascaded, using only the Tx portion of each, 
should reach about 120 dBc rejection, with about 6 dB insertion loss. 
The original two-module circuit  (or maybe even a single one) would 
probably suffice for most applications.

I haven't yet impedance matched the 100 ohm differential filters to 
the 50 ohm cable environment - I'm thinking it may not even be necessary.

If you have any old LAN cards, hubs, or routers around, you may 
already have some nice "free" LPFs for this purpose. The box to put 
them in (packaging cost) is another matter.

Ed


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